1. Field of the Invention
The present invention relates to a molecular drag pump which operates to deliver a gas from the inlet side to the exhaust side under molecular flow conditions, and more particularly to a molecular drag pump composed of a pair of parallel, helically threaded intermeshing rotors for discharging a gas at high speed.
2. Prior Art
There are known various molecular drag pumps in the art. Among the known types of molecular drag pumps is a molecular drag pump which is composed of a pair of parallel, helically threaded rotors intermeshing with each other. Such a molecular drag pump is relatively simple in structure and can be manufactured with ease, but cannot discharge a gas at high speed.
More specifically, the molecular drag pump comprises two parallel intermeshing rotors which are helically threaded in opposite directions and rotatable about their respective axes also in opposite directions. One of the rotors has a right-hand helical rib or thread and a right-hand helical groove, whereas the other rotor has a left-hand helical rib or thread and a left-hand helical groove. These right- and left-hand threads are held in mesh with each other, i.e., positioned in the left- and right-hand grooves, respectively. Since the right-hand groove and the left-hand thread, and also the right-hand thread and the left-hand groove rotate in the same direction in the vicinity of the region where they mesh with each other, the gas delivered from the inlet port to the exhaust port by the helically threaded pump tends to leak from a radial gap which is defined between the meshing threads and grooves. Therefore, the conventional molecular drag pump with helically threaded rotors cannot discharge the gas at high speed.